Drawing semiconductor crystals



Nov. 17, 1964' R. KAPPELMEYER ETAL 3,157,472

DRAWING sEMIcoNnucToR CRYSTALS Filed Sept. 2. 1960 United States Patent O lud 3,157,472 BRAWEIG SEMCGNDUCTGR CRYSTALS Rudolf Kappelmeyer, Heufeld-Waldheim, and Hans- E'riedrich Quast, Freiburg im Breisgau, Germany, assignors to Siemens @a Halslre Aktiengesellschaft, Beriin and Munich, a corporation of Germany Filed Sept. 2, 1966, Ser. N 53,648 Ciaims priority, application Germany Sept. 11, 1959 2 Claims. (Cl. 2li- 301) This invention is concerned with drawing from a melt relatively thin rodlike semiconductor crystals with substantially uniform diameter and substantially free of constrictions normally occurring due to the surface tension of the fluid material of the strand drawn from the melt.

There are several possibilities available for producing highly pure or denitely doped crystalline and particularly mono crystalline semiconductor rods. According to one of the most important of these possibilities, the desired semiconductor is obtained from a gas phase, that is, by thermal decomposition of a highly pure gaseous compound of the corresponding semiconductor, if desired intermixed with highly pure hydrogen, which is caused to crystallize upon a thin rodlike carrier made of the same semiconductor material. This method accordingly requires the use of thin rodlike semiconductor crystals as initial carriers. However, different modes of producing thin rodlike semiconductor crystals are also of interest.

Another important method of producing thin semiconductor crystal rods is the known Czocharlsky method wherein a mono crystal is dipped into a melt and gradually withdrawn therefrom, carrying a strand of molten material along, such strand solidifying successively and forming a rodlike body with a cross-section depending upon the drawing speed. According to the Czocharlslry method proper, the melt may be contained in a Crucible. In accordance with a more e'lcient procedure, the semiconductor material required for the formation of a thin rod can be taken from a molten zone produced, for example, at the end of a sufficiently thick preferably vertically extending rodlike body made of the corresponding pure semiconductor material, whereby the thick semiconductor rod is successively advanced in the direction of the molten zone so as to replenish the melt from which the material is taken by the seed crystal. rhe operation is effected in vacuum or if desired in a hydrogen or argon atmosphere.

As indicated before, the diameter of the fluid material carried along with the seed crystal depends upon the speed at which the seed crystal is withdrawn from the melt and also upon the initial contact area between the seed crystal and the melt. It is entirely possible to draw crystalline rods with a diameter of 3 millimeters or less, by applying appropriate drawing speeds and employing seed crystals of suitably small dimensions. Diaphragms forming apertures may also be used to aid in the formation of the rod diameter.

However, upon producing according to the above explained method semiconductor rods less than 4 millimeters in diameter, trouble is experienced due to formation of undulating periodic constrictions and widenings which stand in the way of obtaining uniformly shaped crystals.

The object of the invention is to eliminate these troubles and, accordingly, to produce more uniform rods with a diameter from a few millimeters down to substantially about one millimeter. t

This is according to the invention accomplished, in connection with a method of drawing thin rodlike semiconductor crystals from a semiconductor melt by using a seed crystal which is dipped into the melt and thereafter gradually withdrawn therefrom, whereby the fluid mate- 3,157,472 Patented Nov. 17, 1964 "ice rial strand carried along with the seed crystal solidies progressively, forming a rodlike body with a cross-sectional area depending upon the drawing speed or, if desired, depending upon the aperture of a diaphragm employed in the operation, by utilizing an electrostatic eld acting upon the fluid material strand carried along by the seed crystal, especially upon the interface between the iiuid and solidified portions of the strand, the strength and orientation of the electrostatic field being with respect to the fluid portion of the material strand such that the operatively effective surface tension thereof is reduced, thereby avoiding periodic constriction of the drawn material.

In order to obtain at the surface of the drawn fluid material a field as uniform as possible, it is recommended to produce the electric ield by means of a direct voltage applied between the drawn material strand and a ring electrode concentrically surrounding the interface between the fluid and the solidified portions and retaining, during the drawing operation, its position relative to the interface. The eld strength to be applied depends upon the surface tension of the duid semiconductor material of the drawn strand. In the case of a semiconductor material exhibiting relatively high surface tension, such for example as silicon, the component of the electric eld extending perpendicular to the surface of the drawn strand shall amount to at least 5 liv/cm. A lower value will sufiice for other semiconductor materials such, for example, as germanium.V

The various objects and features of the invention will appear from the description of an arrangement which will now be described with reference to the accompanying drawing.

Numeral id indicates an evacuated vessel lled with a protective gas, such vessel being provided with suitable means for supervising the interior parts, for example, a suitable window. The desired thin semiconductor rod 1 is drawn from a melt 2 which is at the upper end of a relatively thick vertically extending rod 3, made of the corresponding semiconductor material, produced by an energy source 4, for example, a high frequency coil supplied from a high frequency source 5. A reflector, not shown, may be used to increase the effectiveness of the heating source. For the drawing of the thin rod, there is employed a pointed seed crystal 6 made of the desired semiconductor material, the pointed end of which is in known manner dipped into the melt 2 and thereafter gradually upwardly withdrawn therefrom. The diameter of the thin rod l to be drawn will be a function of the drawing speed. The seed crystal is held in a holder 7 which is moved upwardly in the direction of arrow 9, by means of a gear S, thereby moving it with a given speed away from the melt 2. Further portions of the thick semiconductor body 3 must be melted according to the rate at which the melted zone 2 is consumed, and the body 3 which is mounted in a holder 10 is for this purpose moved upwardly in the direction of the arrow 12, by means of a gear 11, thus moving it progressively into the field of action of the heating zone 4. The advance of the body 3 is advantageously effected under control of a suitable automatic coupling (not shown) governed in accordance with the drawing speed of the seed crystal 6.

The material strand l which is continuously taken from the melt 2 and which upon solidification forms the thin rod, is at the level 13 of the interface between the uid Y ductor material and ythat these troubles can be avoided in v relatively simple-manner by a reduction of the surfaceV tension effected by an electric -field. This field amounts in case of silicon, considering the surface tension applicable therefor, to atleast 5 kv./crn. Lower values can be applied in `connection with other semiconductors. The given value applies for the normal component ofthe field strength with respect to the surface of the material strand 1.

The apparatus described above for. the drawing of thin rods can be modified in known manner. In particular, a molten zone in a crucible can take the place of the molten zone shown. The remaining steps required for the production of the'thin rods correspond in this case to those already described. Upon using in known manner a melt contained in a Crucible, the strand of the material can be guided through the aperturejof a stationary diaphragm which defines the general dimension of the material strand. The present method may be combined with the known method by utilizing ,the electrode, serving in the known method as a measuring electrode and surrounding the material strand, according to the teaching of the present invention, asa carrier of the Yelectric field which reduces the surface tension of the material strand, thus serving Yfor the dimensioning of the electrostatic field.

Changes may be made within theV scope and spirit of the'appended claims which define what is believed to be new and `desired to have protected by Letters Patent.

We claim: Y

1. In theV art of drawing from a semiconductor melt relatively thin rodlike semiconductor crystals with the aid of a seed crystal which is dipped into said melt and thereupon gradually withdrawn therefrom, whereby the strand of liquid material carried along by the seed crystal solidifies successively to form a rodlike crystal with a cross-section depending upon the drawing speed, the method of continuously forming said rodlikecrystal with a substantially uniform diameter, comprising surrounding the liquid material strand, which is being carried along by the seed crystal, with an annular .electrode andapplying a direct voltage between the latter and said strand to produce a substantially uniform electrostatic'fieldY concentrically acting upon said strand substantially at the level of the interface between the liquid and solidified portions thereof, the strength and orientation of said electrostatic feid being with respect to the liquid material strand such that the surface tension is reduced to avoid occurrence of periodic constrictions in the solidified material strand.

2. The method according to claim l, wherein silicon constitutes the material of which the rodlike crystal is to be produced, and wherein the component of the electrostatic field extending perpendicularly to the surface of said material strand amounts at least to 5 liv/cm. Y

References Cited in the file of this patent UNITED STATES PATENTS 2,927,608 Shockley Mar. 1, 1960 

1. IN THE ART OF DRAWING FROM A SEMICONDUCTOR MELT RELATIVELY THIN RODLIKE SEMICONDUCTOR CRYSTALS WITH THE AID OF A SEED CRYSTAL WHICH IS DIPPED INTO SAID MELT AND THEREUPON GRADUALLY WITHDRAWN THEREFROM, WHEREBY THE STRAND OF LIQUID MATERIAL CARRIED ALONG BY THE SEED CRYSTAL SOLIDIFIES SUCCESSIVELY TO FORM A RODLIKE CRYSTAL WITH A CROSS-SECTION DEPENDING UPON THE DRAWING SPEED, THE METHOD OF CONTINUOUSLY FORMING SAID RODLIKE CRYSTAL WITH A SUBSTANTIALLY UNIFORM DIAMETER, COMPRISING SURROUNDING THE LIQUID MATERIAL STRAND, WHICH IS BEING CARRIED ALONG BY THE SEED CRYSTAL, WITH AN ANNULAR ELECTRODE AND APPLYING A DIRECT VOLTAGE BETWEEN THE LATTER AND SAID STRAND TO PRODUCE A SUBSTANTIALLY UNIFORM ELECTROSTATIC FIELD CONCENTRICALLY ACTING UPON SAID STRAND SUBSTANTIALLY AT THE LEVEL OF THE INTERFACE BETWEEN THE LIQUID AND SOLIDIFIED PORTIONS THEREOF, THE STRENGTH AND ORIENTATION OF SAID ELECTROSTATIC FIELD BEING WITH RESPECT TO THE LIQUID MATERIAL STRAND SUCH THAT THE SURFACE TENSION IS REDUCED TO AVOID OCCURRENCE OF PERIODIC CONSTRICTIONS IN THE SOLIDIFIED MATERIAL STRAND. 